Before WordPress 3.0 the importers were in core, and did not include licensing information. It looks like when the GPL licensing info was added to WP & MT importer at least the versioning became explicit. This could affect future compatibility.

No, I'm not on the GPL v1 run here, Denis. From best I can say, lloydbudd's suggestion looks valid to me to just remove the explicit version as it might be misleading and/or introduces compatibility constraints. As far as the license plugin header is anything that counts at all.

The best suggestion I can give is to ask the orginal authors of the code how it was licensed, and then the plugin authors need to decide upon it then.

Next to that, I always suggest to add a legal comment on top of the file(s) as the @license tags or an undocumented plugin header (license is not a defined plugin header) are adding some meta information only. Those might help for a guided guess, but are not solving the general problem that the license information and copyright is still missing.

This form of argumentation might look a bit nit-picking, but compare the license tag to the author tag. As author wordpressorg is named which is not a valid author at all when it comes to licensing/copyright. So obviously, such information is used for some other than the legal purposes. For example to get the plugin listed under such an author who legally spoken most certainly is none.

This is especially noteworthy in this case as the plugins code has been taken out of wordpress core code and a new work has been formed. So licensing applies to a new work in form of the plugin.

So this is another good example why it is advisable to actually have the license information properly documented even ​in or for core (see On Submitting Code). It saves a lot of hassles and users who aquire the code to actually know about the license instead of best guessing only.

As with this case, it's the burden of the ones who compiled the plugin and published it. I can't even say if this is of core scope any longer as this is a plugin on it's own now.